1. Field of the Invention
The present invention relates to a method for heating continuously cast steel slab for production of grain-oriented silicon steel sheet having a grain orientation in the (110) direction as identified by the Miller index and a high magnetic flux density.
2. Description of the Prior Art
In the production of a grain-oriented steel sheet by the steps of slab heating, hot rolling, cold rolling, decarburization and finishing annealing, an indispensable factor in obtaining a secondary recrystallization structure having (110) grain orientation during the finishing annealing step is the existence within the steel structure of a secondary dispersion phase. For this reason, it is necessary for the secondary dispersion phase forming substances such as MnS, AlN and the like to be completely dissolved in solid solution in the matrix in the slab heating step, and then for these secondary dispersion phase forming substances to be finely dispersed in the steel in the hot rolling step.
In order to completely dissolve the secondary dispersion phase forming substances in solid solution during the slab heating step, it is necessary to control the slab heating temperature in a pusher-type or walking-beam type furnace, for example.
In this temperature control, it is important from the point of uniformly heating the slab to properly control the temperature history (temperature vs time) of the portion of the slab which is heated to the lowest temperature (lowest temperature portion).
In the case where the steel slab is heated in a walking-beam type furnace, the contact position between the furnace skids and the lower surface of the slab changes as the slab is transferred through the furnace so that there is no substantial temperature difference between the front side and the back side of the slab and the lowest temperature portion of the slab exists near the center of the slab. Therefore, the temperature of the lowest temperature portion of the slab can be easily controlled by controlling the surface temperature.
On the other hand, when the slab is heated in a pusher-type furnace, the lowest temperature portion of the slab is at or near that part of the lower surface of the slab which is in contact with the furnace skids. It is, however, difficult to directly determine the temperature history of the lowest temperature portion in the pusher-type heating furnace because of structural factors (namely, because the soaking zone has a brick hearth, a dry skid hearth, etc.) and because the lowest temperature portion is always in contact with the furnace skids.
Therefore, slab heating in a pusher-type furnace has conventionally been performed without direct knowledge of the temperature history of the lowest temperature portion of the slab, and the temperature at which such conventional slab heating is to be carried out is determined by such factors as slab surface temperature, the furnace atmosphere temperature, the residence time of the slab in the furnace, the slab extraction pitch, the slab surface temperature after extraction from the furnace and the driving power required in rolling the slab.
In recent years, continuous casting has been widely adopted for production of various grades of steel, and steel slabs for production of grain-oriented silicon steel sheet have also been produced more and more by continuous casting.
Contrary to steel slabs prepared by break-down rolling, continuously cast slabs have an as-cast structure. As a consequence, grain-oriented silicon steel sheet produced from continuously cast slabs has frequently suffered from insufficient secondary recrystallization because of the incomplete dissolution in solid solution of the secondary dispersion phase forming substances into the matrix, and from poor magnetic properties because of abnormal grain growth caused by excessive slab heating.
In spite of the above-mentioned problems peculiar to the heating of continuously cast steel slabs, conventional art methods of heating continuously cast slabs in the pusher-type heating furnace give no consideration to the temperature history of the lowest temperature portion of the slab, namely that part remaining constantly in contact with the furnace skid. Thus, as this lowest temperature portion is more difficult to fully heat than the other portions, it suffers from insufficient dissolution into solid solution of the secondary dispersion phase forming substances such as MnS and AlN in the matrix. As a consequence, secondary recrystallization does not fully develop in the final finishing annealing of the steel sheet made from the slab, and the result is nonuniformity of such magnetic properties as magnetic flux density and iron core loss of the final product.
On the other hand, any attempt to avoid poor secondary recrystallization by raising the temperature of the lowest temperature portion of the slab through control of the conventional factors mentioned above is not likely to be successful since, as mentioned earlier, the continuously cast slab is susceptible to abnormal grain growth so that such abnormal grain growth is apt to occur in portions other than the lowest temperature portion, thus degrading and/or causing nonuniformity in the magnetic properties.
As mentioned above, the conventional method used for slab heating in pusher-type heating furnaces entails problems in that it is apt to cause deterioration and/or nonuniformity in the magnetic properties of the resultant high magnetic flux density, grain-oriented silicon steel sheets.